These studies were designed to determine the potential for various compounds to induce HbF synthesis in experimental animals. Three drugs, 5-azacytidine, cytosine arabinoside, and hydroxyurea, have been studied extensively in rhesus monkey and baboons. 5-Azacytidine is consistently the most active of the three compounds, although significant differences between species and among individuals of a given species have become evident. An individual with severe beta thalassemia, untransfused because of the presence of multiple allo-antibodies, has been treated with 5-azacytidine. An increase in hemoglobin from 5.7 gm/dl to 9.5 gm/dl has been documented after two five day courses of treatment. Therapy is to be continued by the oral route. This represents the first thalassemic patient to achieve significant clinical benefit from the use of 5-azacytidine.